1. Field of the Invention
The present invention relates to the backend processing of integrated circuits. More specifically, the present invention relates to the processing of a topside film of an integrated circuit.
2. Discussion of Related Art
In general, an integrated circuit die has a topside film that is located over an uppermost metal layer. The purpose of the topside film is to protect the uppermost metal layer from damage. Typically, the topside film is a composite film that includes silicon oxide and silicon nitride.
FIG. 1 is a cross sectional diagram of an upper portion of an integrated circuit die 100 that includes silicon substrate 101, intermediate interconnect structure 102, uppermost metal layer 103 and topside film 104. Intermediate interconnect structure 102 includes a plurality of conductive layers, insulating layers and contacts which are configured to route signals to and from circuit elements fabricated in substrate 101. Topside film 104 is located over uppermost metal layer 103. Topside film 104 includes a layer of silicon oxide 105 and an overlying layer of silicon nitride 106. Silicon oxide layer 105 has a thickness on the order of 2000 Angstroms, and silicon nitride layer 106 has a thickness on the order of 8000 Angstroms. Topside film 104 exhibits a first thickness T1 on the sides of uppermost metal layer 103, and a second thickness T2 over the top of uppermost metal layer 103. The first thickness T1 is thinner than the second thickness T2. For example, thickness T1 is typically about 70 percent of thickness T2.
As illustrated in FIG. 2, when die 100 is put in a plastic package, plastic molding compound 200 covers the entire upper surface of die 100. At this point, temperature cycling is commonly used to test the strength of topside film 104. Temperature cycling refers to the process of cycling the packaged die between a low temperature (e.g., -65.degree. C.) and a high temperature (e.g., 150.degree. C.). Due to the different thermal expansion coefficients of plastic molding compound 200 and silicon substrate 101, the molding compound 200 will apply a force on topside film 104. This force is directed radially inward toward the center of die 100 at low temperatures, as indicated by arrow F1. If topside film 104 is not strong enough, topside film 104 will break around the sides of uppermost metal layer 103 as shown in FIG. 3. The force exerted by molding compound 200 is greatest near the outer edge 201 of die 100.
Solutions have been proposed to increase the strength of the topside film at the edges of the uppermost metal layer. One conventional solution is to form sidewall spacers on the sides of the uppermost metal layer prior to forming the topside film.
FIG. 4 is a cross sectional view of a die 400 that includes oxide spacers 401 formed on the sides of uppermost metal layer 103. Because die 400 (FIG. 4) is similar to die 100 (FIG. 1), similar elements in FIGS. 1 and 4 are labeled with similar reference numbers. Silicon oxide layer 405 and silicon nitride layer 406 are formed over uppermost metal layer 103 and sidewall spacers 401. The resulting topside film 404 has a thickness T3 on the sides of uppermost metal layer 103 that is significantly greater than the thickness T1 on the sides of uppermost metal layer 103 (FIG. 1). As a result, topside film 404 is stronger than topside film 104 (FIG. 1). However, the formation of sidewall spacers 401 significantly increases the complexity of the process used to create die 400.
It would therefore be desirable to have a method and structure for increasing the width, and therefore the strength, of the topside film on the sides of an uppermost metal layer without increasing process complexity.